1. Field of the Invention
The invention relates to a body formed by assembling a plurality of units and used in particular to filter particles contained in the exhaust gases of an internal combustion engine, in particular of the diesel type.
Conventionally, before being exhausted to the atmosphere, exhaust gases may be purified by means of a particle filter such as the prior art filter shown in FIGS. 1 and 2.
FIG. 1 shows a particle filter 1 in cross section taken along the line B-B in FIG. 2 and FIG. 2 shows the particle filter 1 in longitudinal section taken along the line A-A in FIG. 1.
2. Description of the Related Art
The particle filter 1 conventionally includes at least one filter body 3 inserted into a metal enclosure 5. The filter body 3 is produced by assembling and machining a plurality of units 11, referenced 11a-11i, produced by extruding a ceramic material (cordierite, silicon carbide, etc.) to form porous honeycomb structures.
Before assembly and machining, a unit 11 (see FIG. 3) conventionally has the shape of a rectangular parallelepiped extending along an axis D-D between a substantially square upstream face 12 and a substantially square downstream face 13 onto which a plurality of rectilinear square section passages 14 parallel to the axis D-D discharge.
The exterior faces 15 of the passages 13p located at the periphery of the unit 11, referred to as “peripheral passages”, form an exterior surface 16 of the unit 11 (see FIG. 3). Because of the substantially square cross section of the unit 11, the exterior surface 16 has four faces 16a-16d, pairs of which are mutually perpendicular.
After extrusion, the units 11 are alternately blocked at the upstream face (outlet passages 14s) or the downstream face (inlet passages 14e). This is known in the art.
The units 11a-11i are then assembled by bonding them together by means of ceramic cement joints 17 generally consisting of silica and/or silicon carbide and/or aluminum nitride. The resulting assembly can then be machined to a round section, for example. This produces a cylindrical filter body 3 with axis C-C which can be inserted into the enclosure 5, a peripheral seal 18 that is impermeable to the exhaust gases being disposed between the exterior filter units 11a-11h and the enclosure 5.
As shown by the arrows in FIG. 2, the flow F of exhaust gases enters the filter body 3 via the inlet passages 14e, passes through the filter walls 20 of those passages into the outlet passages 14s, and is then exhausted to the exterior.
After a certain time of use the performance of the engine is degraded by particles or “soot” accumulated in the inlet passages 14e of the filter body 3. For this reason, the filter body 3 must be regenerated regularly, for example every 500 kilometers. Regeneration or “unclogging” consists in oxidizing the soot by heating it to a temperature at which it can ignite.
During regeneration phases, the exhaust gases carry in the downstream direction all the heat energy given off by the combustion of the soot. Moreover, because the soot is not deposited uniformly in the various passages, the combustion areas are not uniformly distributed in the filter body 3. Finally, the peripheral areas of the filter body 3 are cooled by the surrounding air through the metal enclosure 5.
As a result of this the temperature differs in the various areas of the filter body 3 and does not vary uniformly. The non-homogeneous temperatures in the filter body 3 and the different kinds of materials used for the filter units 11a-11i, on the one hand, and the joints 17, on the other hand, generate high local stresses that can cause local breaks or cracks. In particular, the local stresses at the interfaces between the units 11a-11h and the enclosure 5 and between the units 11a-11i and the joints 17 may lead to the units 11a-11i becoming unstuck, thereby reducing the service life of the particle filter 1.